The present invention relates to a hydraulic brake system with slip control for automotive vehicles. The system incorporates a master cylinder actuatable by a brake pedal directly or by the intermediary of a power booster. Valve means are provided between the master cylinder and the wheel brakes connected to the master cylinder through which pressure fluid can be removed from the brake circuits which can be subsequently replenished out of an auxiliary-pressure source. The stroke limitation of the brake pedal is performed during slip control.
A hydraulic brake system with the preceding features is known from German published patent application No. 30 40 562. This known system comprises a hydraulic brake power booster wherein a pressure of the booster proportional to the actuating force can be brought about through a pedal-actuatable brake valve. The hydraulic power booster permits actuation of a tandem master cylinder, to whose working chambers wheel brakes are connected. The connecting lines between the working chambers of the tandem master cylinder and the wheel brakes connected thereto can be interrupted by electromagnetically actuatable valve means. In addition, allocated to the wheel brakes are further valve means by which, in case of need, pressure fluid can be removed from the wheel brakes so that the effective braking pressure decreases which, as a result on occurrence of an imminent locked condition, causes the respective vehicle wheel to be re-accelerated sufficiently.
Pressure fluid which was removed from the wheel brakes during brake slip control is replenished out of the pressure chamber of the hydraulic power booster, the pressure fluid prevailing in the pressure chamber of the hydraulic power booster being supplied through additional valve means by way of the sleeves of the master cylinder pistons into the corresponding working chambers. With a view to safeguarding a minimum fluid volume in the tandem master cylinder, it is necessary in the known brake system that stroke limitation takes place. For this purpose, in the event of pressurization of the working chambers of the tandem master cylinder, a so-called positioning tube is simultaneously pressurized in the brake release direction in opposition to spring force, whereby the positioning tube moves into abutment on a stop of the booster piston and, due to the ratios of effective surfaces prevailing, prevents further displacement of the booster piston and of the master cylinder pistons.
As a whole, this known brake system is of relatively complicated design which is above all due to the booster piston being partially guided in the positioning tube, while, in turn, the outer periphery of said positioning tube is slidably arranged in the housing.
A brake system has also been proposed (P No. 33 38 249.2), wherein the end surface of the master cylinder piston facing the working chamber is larger than the effective surface of the booster piston and wherein a stepped piston is used as a master cylinder piston, an annular surface of the master cylinder piston being applicable with the pressure prevailing in the working chamber. Such a design permits the effective surface of the master cylinder piston to be reduced during normal braking actions, while valve means are provided allowing discontinuation of the pressurization of the annular surface of the master cylinder piston. Upon change-over of the valve means, the effective surface of the master cylinder piston will increase so that a resetting force will be exerted on the master cylinder piston, that is on the brake pedal, when the working chamber of the master cylinder is connected to the pressure chamber of the hydraulic power booster. For the valve means, an electromagnetically or hydraulically controllable two-way/two-position directional control valve is used.
It is the object of the present invention to design the braking pressure generator such that all necessary valves either are accommodated within the unit or are simplified to such extent as to allow reduction of the manufacturing costs of the brake system.